I have a Philadelphus. I think it was a "snowball" or something like that. It has a double flower. I planted it years ago on the southwest corner of my house in the eastern suburbs of Seattle (a bit colder in winter, warmer in summer). It has never flowered well at all. It is big, 15 feet or so and it looks healthy, but very few blooms. Could it be too damp? Too fertile? It is definately a Philadelphus, that is not in question.
A prominent one is 'Minnesota Snowflake'. Non- or shy-flowering mockoranges are right up there with non-flowering wisterias: many posts on the internet asking about these. Haven't seen any solid-sounding universally applicable explanations.
